The invention disclosed herein pertains generally to shaft-turning devices and more particularly to an arrangement for turning a turbine shaft.
After a turbine has been shut down, it is still necessary to keep the turbine rotor shaft turning at a slow rate until the rotor has cooled off completely in order to prevent the shaft from warping. When starting the turbine it is again necessary to turn the rotor shaft in order to gradually overcome the inertia of the machine.
Prior art shaft-turning devices, used for rotating turbine shafts, usually include a system of rods which slide vertically and which are moved by a drive mechanism. A bolt or pawl or latch, which is connected to the system of rods, engages the teeth of a gear, which gear is fixedly connected to a turbine shaft. By rotating the gear teeth the pawl and the system of rods turn the turbine shaft. In such prior art shaft-turning devices the system of rods may be either pushed or pulled in the vertical direction. This is disclosed in the Swiss Pat. No. 592,826.
The prior art shaft-turning devices referred to above have the disadvantage that the teeth of the gears connected to the turbine shafts, as well as the sections of the pawls which engage these teeth, are angular, and that in most instances the engaging sections of the teeth of the gears have a concave shape. Where the gear teeth of shaft-turning devices have such a concave shape, the engaging pawls have either a convex or a circular shape. If the upstroke of a pawl is improperly set, or the gear continues to turn after the pawl upstroke, there is a danger that either the gear teeth and the pawl will not mesh or that the pawl will strike the top of a tooth during a subsequent upstroke. Because great forces must be applied to rotate a turbine shaft, the abovementioned situations may result in damage to the engaging sections or even to a breaking of the gear teeth. There is even a danger of destroying the turning device should a ratchet wheel rotate backwardly. Such a situation may arise with compressor drive shutdowns where the torque exerted by the backward turning ratchet wheel overrides the turning force of the pawl.
Accordingly, a primary object of the present invention is to provide a shaft-turning device having a ratchet wheel and a pawl whereby the pawl may engage the teeth of the ratchet wheel in any position, even if the ratchet wheel is rotating backwardly, without any damage to the pawl or to the teeth of the ratchet wheel.
Apparatus for rotating a shaft of a turbine, according to the present invention, includes a ratchet wheel having teeth with bearing surfaces which have convex roll-off curvatures. A pawl which engages the teeth of the ratchet wheel has a contact surface with a convex roll-off curvature similar to that of the teeth. In addition, the pawl includes a flat upper surface which merges into the contact surface, and a snout-like projection which extends from the contact surface.
By giving each tooth of the ratchet wheel a convex roll-off curvature, the bearing surface of each tooth is made to extend from a base of the tooth to a top of the tooth. This is an advantage of the present invention because the turning force exerted by the pawl may then be applied at any point over the full length of each tooth without causing any damage. Furthermore, in the event of a faulty engagement between the pawl and a ratchet tooth, the shapes of the pawl and the tooth permit the pawl to slide into engagement with a succeeding tooth without incurring any damage.
A further advantage of the present invention is that the shape of the pawl prevents it from jamming into or grating a tooth base, which jamming and grating could otherwise lead to damage produced by frictional heating.